在QWidget内的QGraphicsScene内绘制

Question

I am trying to have a window (in the form of a QWidget ) consisting of both a menu on the right and a graphics area on the left. Despite the numerous websites explaining the many ways to use QGraphicsScene and QGraphicsView , I just couldn't figure out how to do it.

Here's main.cpp modified to work on its own :

#include <QApplication>
#include <QRectF>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QPushButton>
#include <QVBoxLayout>
#include <QGraphicsRectItem>
#include <QPalette>

int main (int argc, char * argv []) {
    QApplication app (argc, argv) ;

// Main window
    QWidget frame ;
    frame.setFixedSize(750, 550) ;
    frame.show() ;

// Right side, works ok
    QWidget menu (&frame) ;
    menu.setFixedSize(200, 500) ;
    menu.move(550, 10) ;
    QPalette pal = menu.palette() ;
    pal.setColor(QPalette::Background, Qt::red) ;
    menu.setPalette(pal) ; // I expected this to color the whole area 
    // to show the extent of the zone devoted to the menu,
    // but it only outlines the button
    menu.show() ;
    QPushButton button ("Menu", &menu) ;
    button.show() ; // I didn't think this was necessary,
    // but the button didn't appear without this line

// Left side, nothing displayed
    QVBoxLayout layout ;
    QGraphicsScene * scene = new QGraphicsScene () ;
    QGraphicsView view (scene) ;
    layout.addWidget(&view) ;
    QWidget canvas (&frame) ;
    canvas.setLayout(&layout) ;
    // I found this trick to include a QGraphicsScene inside a QWidget,
    // I haven't had the opportunity to see whether it really works.
    scene->addItem(new QGraphicsRectItem(10, 10, 20, 20)) ;
    // The above line has no visible effect
    view.show() ;

    return app.exec() ;
}

I would expect this to create a window, put a bunch of buttons on the right side (or in the case of the rediced code I provided, just a single button), and draw a rectangle on the left, but it leaves the whole left area blank. Does the problem come from how I put the QGraphicsView inside the QWidget ? Or is it failing to draw because of something else ? Do I have to update the QGraphicsView to reflect the change ? Is it just out of visible range ? Finally, is the failure to draw in any way related to the fact that the whole application crashes on line QWidget canvas (&frame) ; when closed ?

Answer 1

I just repaired you program a bit, so that it illustrates, what you can do with Qt and how you should likely use the framework.

I just moved the QPushButton to a QAction residing in a QMenuBar. The QMenuBar can be added to a QMainWindow, which is reasonable for a normal app.

The central widget of the QMainWindow contains the QGraphicsView. Now, you just forgot to connect the QGraphicsScene with the QGraphicsView. That was the reasons for not seeeing anything in your view.

QGraphicsView and QGraphicsScene are just a typical example for a MVC pattern. You can also add another QGraphicsView and connect it to the same QGraphicsScene.

You should also create all you objects with new, as Qt automatically disposes all its children of a QObject, if it is either deleted or leaves scope.

If you are realyl interesting into seriously learning Qt I suggest, that you are creating plenty small example programs like these. It really helped me a lot.

#include <QApplication>
#include <QMenuBar>
#include <QGraphicsView>
#include <QVBoxLayout>
#include <QGraphicsRectItem>
#include <QMainWindow>

int main(int argc, char* argv[]) {
    QApplication app(argc, argv);

    auto mainWindow = new QMainWindow;
    auto menuBar = new QMenuBar;
    auto menu = new QMenu("Menu");
    auto action = new QAction("Action");
    menu->addAction(action);
    menuBar->addMenu(menu);
    mainWindow->setMenuBar(menuBar);

    auto frame = new QFrame;
    frame->setLayout(new QVBoxLayout);
    mainWindow->setCentralWidget(frame);

    auto scene = new QGraphicsScene();
    auto view=new QGraphicsView(scene);
    view->setScene(scene); // That connects the view with the scene
    frame->layout()->addWidget(view);

    QObject::connect(action, &QAction::triggered, [&]() {
        scene->addItem(new QGraphicsRectItem(10, 10, 20, 20));
    });
    mainWindow->show();

    return app.exec();
}